Polymethylaluminoxane, which is traditionally formed from the reaction of trimethylaluminum and a source of water, is used as a catalyst component with a metallocene catalyst component in olefin polymerizations. As indicated in European Patent Publication No. 393,358, it is difficult to form homogeneous polymethylaluminoxane solutions having good storage stability due to precipitation or gel formation in such solutions upon storage. This European patent publication mentions the reaction of polymethylaluminoxane with a C.sub.3 to C.sub.9 branched alkylaluminum compound in organic solvent to yield a homogeneous solution. A representative alkylaluminum compound which can be used is triisobutylaluminum.
An earlier disclosure of modified polymethylaluminoxane containing C.sub.2 or higher alkyl groups, which has enhanced solubility in organic solvent is European Patent Publication No. 372,617 which describes compositions containing certain alkyl groups, e.g., isobutyl, n-butyl, and n-hexyl groups. This earlier European patent publication (which corresponds to U.S. Pat. No. 5,041,584) does not specifically address the issue of enhancing the storage stability of conventional polymethylaluminoxane solutions which are not so modified.
A more recent U.S. patent which describes the use of tri-n-alkylaluminum compounds, having from 2 to 20 carbon atoms in the alkyl groups, to aid in the solubilization of polymethylaluminoxane in hydrocarbon solvent is U.S. Pat. No. 5,066,631 to S. A. Sangokoya.
All of the above references focus upon the use of organoaluminum compounds as either reagents or additives to yield homogeneous polymethylaluminoxane solutions.